There is a known internal combustion engine in which an exhaust gas sensor that produces an output according to the amount or concentration of a specific component in exhaust gas, for example, an oxygen concentration sensor that produces an output according to the oxygen concentration in exhaust gas, is disposed in an engine exhaust passageway, and the air-fuel ratio is controlled on the basis of the output voltage of the oxygen concentration sensor. However, the output of the foregoing exhaust gas sensor changes according to the temperature of the sensor element.
To overcome this, the oxygen concentration sensor is provided with an electric heater, which is used to heat the sensor element so that the temperature of the sensor element is kept at a predetermined target temperature, in a known internal combustion engine (see Japanese Patent Application Publication No. 2005-2974 (JP-A-2005-2974)). In this technology, the temperature of the sensor element is represented by the impedance of the sensor element. Therefore, a target impedance that represents the target temperature is set beforehand, and the actual impedance of the sensor element is detected, and the output of the electric heater is controlled so that the actual impedance becomes equal to the target impedance.
However, the impedance of the sensor element changes according to the property of the fuel used in the engine. Specifically, in the case where the fuel used in the engine is, for example, an alcohol-containing gasoline, the impedance of the sensor element changes according to the alcohol concentration in the fuel. This is because the components of exhaust gas vary according to the alcohol concentration in the fuel, and, for example, the electric resistance of an electrolyte portion formed from zirconia changes according to the components contained in the exhaust gas. Therefore, even though the actual impedance is maintained at the target impedance, there is a possibility that the actual temperature of the sensor element may not be maintained at the target temperature depending on the property of the fuel used in the engine. This means that the output of the exhaust gas sensor is not necessarily accurate.